Steroid profiles of the female tilapia, Oreochromis mossambicus, and correlation with oocyte growth and mouthbrooding behavior.

Plasma levels of progesterone, 17 alpha-OH-progesterone, testosterone, and estradiol-17 beta were measured by radioimmunoassay during the ovarian cycles of two groups of female tilapia, Oreochromis mossambicus. One group included females that successfully mouthbrooded fry, while the other group consisted of females in which the zygotes were either removed or swallowed within 1 day after spawning. The mouthbrooders had a longer ovarian cycle (about 40 days) and were sampled 1, 3, 5, 7, 10, 15, 20, 25, 30, 35, and 40 days after spawning. The non-mouthbrooders had an ovarian cycle of about 25 days. They were sampled 3, 5, 7, 10, 15, 20, and 25 days after spawning. Initial peaks in levels of testosterone, estradiol-17 beta, and progesterone occurred later in the cycle of mouthbrooders. The first peak of testosterone and estradiol-17 beta occurred at 15 days after spawning. While estradiol-17 beta levels remained high, testosterone levels fell at 25 days after spawning, and increased again just prior to spawning. In the latter phase of mouthbrooding (15-25 days after spawning), the oocytes in the ovary did not increase in size, and testosterone and estradiol levels were high. During this time, estradiol may have a function other than stimulating vitellogenesis, such as an involvement (with testosterone?) in parental behavior, or protecting the oocytes from atresia. In non-mouthbrooders, testosterone, estradiol-17 beta, and progesterone levels initially peaked at 10 days after spawning, then dropped at 15 days after spawning. At the end of the cycle, testosterone and estradiol-17 beta levels increased again. The drop in estradiol levels is contrary to the profile seen in mouthbrooders. Also in mouthbrooders, progesterone levels did not rise until 25 days after spawning, then decreased and peaked again towards the end of the cycle. 17 alpha-OH-progesterone concentrations were low, with a single peak at 7 days after spawning in non-mouthbrooders, and at 40 days after spawning in mouthbrooders. There appears to be a relationship between the delayed initial peaks of the steroid hormones measured, oocyte growth arrestment, and longer-lived postovulatory follicles in mouthbrooding female tilapia.